The organic thin film solar cell is an all solid type thin film solar cell containing an organic semiconductor and thus has been expected to be large in area and produced by an inexpensive manner, as well as light in weight and rich in flexibility. However, a significant increase in the conversion efficiency of the organic thin film solar cell has become an important issue towards the practical realization of the cell, and as the result, researches and development therefor have been vigorously carried out around Europe and the United States.
The fullerene derivative is an organic semiconductor material that is high in electron accepting properties and has been expected to be applied to an organic photoelectric conversion device (organic solar cells, light sensors). A methanofullerene wherein phenyl and butyric acid ester groups are cross-linked with methylene (phenyl-C61-butyric acid methyl ester; PCBM) is a widely known fullerene derivative for an organic thin film solar cell, and an improvement in energy conversion efficiency by 5 percent was achieved by an organic thin film solar cell of bulk heterojunction structure comprising a mixed active layer of the PCBM and a conjugated polymer, i.e., poly(3-hexylthiophene: P3HT) (see Non-Patent Literatures 1 and 2).
The energy conversion efficiency is represented by the formula “conversion efficiency (η)=open circuit voltage (VOC)×short circuit current (JSC)×fill factor (FF)”.
As apparent from the formula, an increase in open circuit voltage contributes significantly to an increase in conversion efficiency. Furthermore, it is known that the open circuit voltage correlates strongly to the energy difference between the HOMO (highest occupied molecular orbital) of a donor and the LUMO (lowest unoccupied molecular orbital) of an acceptor (Non-Patent Literature 3), and thus an increase in the LUMO energy of an acceptor material leads to an increase in open circuit voltage. Specifically, Blom et al have reported that bismethanofullerene (bisPCBM) where two phenyl butyric acid ester groups were substituted was increased in LUMO energy by about 100 meV compared with PCBM (by first reduction potential measurement) and thus was increased in open circuit voltage by 0.15 V and had 1.2 time of photoelectric conversion efficiency (Non-Patent Literature 4).
Non-Patent Literature 5 is an example which refers to the LUMO energy level of an acceptor material and has designed an acceptor having a higher LUMO energy than PCBM, but exhibited an open circuit voltage of 0.65 V when formed into a device with P3HT, which open circuit voltage cannot be regarded as a significant improvement.
Meanwhile, in Non-Patent Literature 6, a study was carried out about the correlation of open circuit voltages of various methanofullerene derivatives produced by introducing substituents into the phenyl group of PCBM, but these compounds have methanofullerene derivative structures where the substituents are substituted for the fullerene nucleus via a methylene bridge and thus the effect of the substituents is indirect with respect to the fullerene nucleus. There is, therefore, a certain limit to enhance the LUMO energy.
As described above, these reports are generally insufficient as guidelines to obtain higher open circuit voltage and thus insufficient to solve the problems.